Photochromic substances are generally chemical compounds which undergo a reversible cycle of coloration and color extinction upon exposure respectively to ultraviolet light (200 to 400 nm) and then to either visible light, infrared light or heat. A number of such compounds are known in the art, and a variety of commercial uses are known or have been proposed.
Because there is no necessity for developing the image as in conventional silver halide photography, or in various electrophotographic procedures which depend upon the use of photoconductive materials and the deposition of toner particles, photochromic reactions have found special utility in dry photography, halograms, radiation dosimeters and the like. They have recently been proposed for use in data display units, photomasking techniques and in camouflage applications.
Advances in commercial applications for photochromism have been retarded for a number of reasons. One of these is that the most useful photochromic compounds, the spiropyrans and the mercury bis dithizonates are complex organic compounds which are difficult and expensive to produce. Another is that the sensitivity of presently known photochromic systems is low, and that they will undergo only a limited number of photochromic cycles, that is, cycles of alternate photocoloration and photoextinction.
The art, therefore, has long sought methods to produce photochromic systems which are inexpensive to prepare, of high sensitivity, and capable of undergoing a large number of photochromic cycles.